This invention relates to a high frequency heating apparatus wherein a DC voltage obtained by rectifying in a rectification circuit a high frequency voltage produced by an inverter circuit is applied to a magnetron, thereby driving the same, and more particularly to such a high frequency heating apparatus provided with compensation means for compensating an anode current of the magnetron so that it is maintained at a predetermined value.
A conventional high frequency heating apparatus of the above-described type such as a microwave oven comprises rectification means for converting a commercial AC power supply voltage to DC voltage and inverter means including a switching element and a transformer which also serves as inductance coil means. The DC voltage from the rectification means is applied to the transformer through the switching element turned on and off so that a high frequency voltage is obtained at the inverter means. The high frequency heating apparatus further comprises a magnetron connected to a high frequency rectifying circuit through which the DC voltage from a transformer secondary winding is applied to the magnetron and a control circuit for controlling "on" and "off" periods of the switching element so that the magnetron anode current is controlled. The inverter means is provided for the purpose of reducing the size of the transformer employed to obtain a high voltage which is applied to the magnetron.
In accordance with the above-described high frequency heating apparatus, the magnetron is oscillated upon application of the DC voltage thereto. As a result, microwave energy is generated by the magnetron and radiated to foodstuff, which is cooked. When the magnetron is continuously driven, a permanent magnet of the magnetron is heated as the result of generation of heat by the magnetron. Consequently, the number of magnetic fluxes induced is reduced with the increase in the temperature of the permanent magnet. An amount of anode current of the magnetron is increased with the temperature increase of the permanent magnet and accordingly, an amount of the high frequency energy generated is also increased therewith. In order to prevent the amount of the magnetron anode current from being uselessly increased, a current transformer is conventionally provided at the primary winding side of the transformer for detecting a transformer input current and current control means is provided for controlling the switching element in response to a current detection signal from the current transformer so that the magnetron anode current is maintained at a predetermined value. Alternatively, the current transformer is provided at the secondary winding side of the transformer. However, these conventional arrangements for solving the above-described problem necessitates a current transformer which is expensive to be produced. Furthermore, the provision of the current transformer at the primary or secondary winding side of the transformer complicates the construction of insulation between the current transformer and transformer, which also increases the production cost of the high frequency heating apparatus.